The photopolymerizable composition is a material which undergoes changes in the physical properties of the composition upon polymerization induced by light irradiation and is widely used in the fields of printing, printed circuit, fine processing such as ultra LSI, painting, ink, hologram recording and three dimensional molding, thus the application of the composition is more and more expanding. The photopolymerizable composition fundamentally comprises an addition-polymerizable, ethylenically unsaturated compound and a photopolymerization initiator. Since this kind of composition undergoes polymerization upon light irradiation and thereby is cured and insolubilized, a photosensitive composition is prepared by adding a binder resin having a film-forming ability and a heat polymerization inhibitor, if desired, to the photopolymerizable composition to form an appropriate coating and subjected to light irradiation through a desired negative image and only the non-irradiated area is removed by a proper solvent (hereinafter referred simply as "development") to form an objective cured image. Such an image formation method is well known very useful in preparing a printing plate.
The photopolymerization initiator used in the polymerizable composition is conventionally benzyl, benzoin ether, Michler's ketone, anthraquinone, acridine, phenazine or benzophenone. However, the composition comprising such a photopolymerization initiator is low in the sensitization rate and extremely poor in the photopolymerization ability to visible light at 400 nm or more as compared with the photopolymerization ability to the light in an ultraviolet wavelength region of less than 400 nm. Accordingly, the photopolymerizable composition comprising a conventional photopolymerization initiator is strictly limited on the application range thereof.
A photosensitive composition able to be highly cured by a small amount of light irradiation has been long demanded. Such a photosensitive composition is useful as a photosensitive material suitable, for example, for the projection exposure of non-contact type. Further, in recent years, the image information technique using a visible laser has come into a practical use and accordingly, a photosensitive composition having a high photopolymerization ability to visible light has been keenly demanded. The visible laser light which is very expected to be use in such a method is an Ar.sup.+ laser light having a wavelength at 488 nm, YAG--SHG laser light having a wavelength at 532 nm, or He--Ne laser light having a wavelength at 543 nm.
With respect to the photopolymerizable composition comprising a photopolymerization initiation system sensitive to visible light, several proposals have been offered. For example, U.S. Pat. No. 2,850,445 has reported that certain kinds of photosensitive dyes such as Rose Bengale, Eosine and erythrosine exhibit effective sensitivity to visible light. Also, in improved techniques, a composite initiation system of a dye and an amine (see, JP-B-44-20189 (the term "JP-B" as used herein means an "examined Japanese patent publication")), a system of a hexaarylbiimidazole, a radical generating agent and a dye (see JP-B-45-37377), a system of hexaarylbiimidazole and p-dialkylaminobenzylidene ketone (see, JP-B-47-2528 and JP-A-54-155292 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), a system of a 3-ketocoumarin compound and an active halogen compound (JP-A-58-15503) and a system of a substituted triazine and a merocyanine dye (see JP-A-54-15102) have been proposed. These techniques are effective to visible light but in practical viewpoint, the sensitivity achieved is still insufficient. JP-A-2-244050 proposes a system comprising a dye having a 4-thiazolidinone skeleton and a radical generating agent as a highly sensitive initiation system. This system is highly sensitive, however, the maximum photosensitivity is in the wavelength region of less than 500 nm and thus the photosensitivity is not sufficient to light at 500 mm or more.